RP11 - Reactions of Metal-Aqua Ions
Lajoy Tucker & Dr. Davinder Bhachu
Teachers
Contents
Introduction to Metal-Aqua Ions
Metal-aqua ion: A complex ion in which a transition metal ion is surrounded by water molecules acting as ligands (e.g. ).
Ligand: A species that forms a dative covalent bond to a central metal cation.
Ligand substitution: A reaction where one or more ligands in a complex ion are replaced by different ligands.
Precipitation reaction: Occurs when ions are added to metal-aqua ions, forming insoluble metal hydroxides.
Reactions of Metal Aqua Ions Explained
Basic Principles
Transition metal ions in aqueous solution typically exist as hexa-aqua complexes:
.
These ions show acidic behaviour, and undergo:
Hydrolysis (water ligands donate their protons).
Precipitation with bases (e.g. , ).
Ligand substitution with excess or concentrated .
Theory and full details of these reactions covered in ‘Reactions of ions in aqueous solution’ topic
Practical Method
Aim: To investigate the reactions of aqueous metal ions with hydroxide ions, ammonia, carbonate ions, and chloride ions, and observe changes in colour and solubility.
Apparatus & Reagents:
Test tubes, test tube rack
Pipettes / droppers
Spatula
Distilled water
Metal salts: e.g.
Reagents:
Procedure (repeat steps for each metal ion solution):
If solid salts provided, add a spatula of the salt to a test tube and completely dissolve in distilled water
A. Reaction with Sodium Hydroxide ()
1. Add 10 drops of the metal ion solution to a test tube.
2. Add NaOH solution dropwise, shaking gently between additions.
3. Observe and record the colour change or precipitate.
4. Add until in excess (approx. 10 more drops) and observe if the precipitate dissolves.
5. Record all observations.
B. Reaction with Ammonia ()
1. Use a fresh sample of the metal ion solution.
2. Add dropwise, recording the colour and whether a precipitate forms.
3. Add excess and observe whether the precipitate dissolves (indicating ligand substitution).
4. Record all observations.
C. Reaction with Sodium carbonate ()
1. Use a fresh sample of the metal ion solution.
2. Add solution dropwise, recording the colour and whether a precipitate forms.
3. Add excess ₃ and observe whether the precipitate dissolves (indicating ligand substitution).
4. Record all observations.
D. Reaction with Concentrated
1. Add metal ion solution to a test tube.
2. Add a few drops of conc. , swirl gently.
3. Observe any colour change
4. Keep adding until no further change
5. Record the colour changes and final solution appearances.
Summary of Expected Observations
With bases (,,)
With HCl
Blue solution Yellow-green solution
Sample Equations
Dropwise (no further reaction on excess):
Dropwise
Excess
Dropwise (no further reaction on excess):
Recording Observations
Ensure both the original and final appearances are clearly stated including changes in both colour and state.
E.g.
Blue solution to blue-green precipitate
No answer provided.
Worked Example
Q: Describe and explain the observations when NaOH is added dropwise and then in excess to Al³⁺ solution. Include relevant chemical equations in your answer.
Dropwise: Colourless solution to white precipitate
Excess: White precipitate dissolves to form a colourless solution
Equations:
Dropwise:
Excess:
Describe what you would observe when aqueous is added dropwise and then in excess to a solution of . Include the ionic equation for the reaction. (3 marks)
Answer
Mark Scheme:
Green solution to green precipitate [1]
No further change in excess NaOH [1]
Ionic equation:
Key Tips & Reminders
Know the difference between precipitation and ligand substitution.
Redissolving behaviour is key for amphoteric hydroxides like .
Always write balanced equations with state symbols. Spectator ions (e.g. do not need to be included)
Avoid vague answers like “colour change” - be specific!
No answer provided.